A tease of Valve's Steam mobile app is now available. Right now you can't properly use it, as attempting to login just adds you to the huge list of people wanting a beta invite, but the idea is there, the foundation has been laid.
Steam for iOS, Android lets you do virtually everything the proper Steam application does, apart from playing games. The description on the Android Market says:
Steam: chat and participate in the Steam community, catch unbeatable Steam sales
With the free Steam app for Android, you can participate in the Steam community wherever you go. Chat with your Steam friends, browse community groups and user profiles, read the latest gaming news and stay up to date on unbeatable Steam sales.
Is this a move by Valve to enter the mobile market? Could we see Steam-powered games on iOS and Android devices? One can wish, one can wish.